The present invention relates to integrated circuit structures and fabrication methods.
1. Background: Metallization
Aluminum has been the predominant metallization system in integrated circuit processes since the 1970s. However, pure aluminum metallization suffers from electromigration and/or spiking, and hence alloys are used rather than pure aluminum.
Electromigration is a phenomenon wherein, at high current densities, aluminum leads will develop necked-down portions or even voids. Unfortunately, these events usually take place after the chip has left the factory and is in operation in the field, causing a failure of the chip. Aluminum is normally alloyed with copper and/or titanium to reduce electromigration. However, copper is difficult to etch, and therefore the fraction of copper alloying is normally kept to about 1% or less. (Otherwise residues can be left behind after the etch process.) Higher concentrations of copper would be even more effective for reducing electromigration, if the etching problems could be solved.
Another important limitation on aluminum is that where pure aluminum directly contacts silicon, some of the silicon may be dissolved into the aluminum metallization when heating occurs. This can lead to the aluminum growing into the substrate to "spike" (create short-circuits through p/n junctions). Aluminum layers which must contact silicon are commonly alloyed with 1/2% to 1% of silicon to reduce junction spiking.
2. Background: Prior-Art Anti-Electromigration Techniques
Additional techniques for increasing the resistance to electromigration failure of an interconnect process include the following:
Using layered Al film structure, with a highly electromigration resistant metal (such as Ti, W, or Mo) as the central layer of a tri-layer film. PA1 planarizing the intermetal dielectric, to eliminate thinning of the conductor lines as they cross steps. PA1 selectively depositing a layer of CVD W over the Al lines. PA1 avoiding the use of AlSi when fabricating narrow, multi-level-metal structures. PA1 replacing the Al metallization with a more electromigration-resistant metal, such as W or Mo. PA1 metallization has reduced susceptibility to electromigration; PA1 simple fabrication process; PA1 easier to etch than large homogenous layer; PA1 distribution of alloy can be adjusted by anneal; PA1 allows use of higher concentrations of alloy.
Metallization processes in multi-level interconnect applications involving aluminum alloys are further discussed in, for example, U.S. application Ser. No. 60/044,523 filed Apr. 22, 1997 (attorney's docket no. TI-23021) and Ser. No. 60/037,123 filed Feb. 3, 1997 (attorney's docket no. TI-23072), both of which are copending and commonly owned with the present application, and are hereby incorporated by reference.
Application TI-23021 suggests, among other embodiments, an embodiment in which the use of two stages of metal deposition permits the creation of differing aluminum alloy compositions in the contact or via hole and on the surface of the dielectric. Preferably, the aluminum alloy in the contact has a higher percentage of silicon (or germanium) and of copper than the aluminum on the surface.
Application TI-23072 suggests, among other embodiments, an embodiment in which copper is used as a wetting layer (for hole filling) in an aluminum damascene process. The local introduction of copper near the contact provides the best electromigration resistance at the locations of the highest current density.
Additional Definitions and Background
"Joule heating" refers to the dissipation of power when a current I is flowed through a resistance of value R. For DC current, the power dissipated is I squared times R. In analyzing thin film metallization, the resistance can be expressed as a resistance per unit length, and this formula can then be used to derive the power dissipated per unit length. The relevance of electromigration to joule heating is that, when a particular section of a thin film metallization line begins to become thinner in service, the localized heating in this particular thinned down section of the metal line will increase. This increases the diffusion rate of the metal atoms. This in turn accelerates the electromigration process.
"Current crowding" refers to the nonuniformity of current distribution in a solid conductor. For example, when a thin film metallization line has a neck down portion in it, the density of current (per unit cross sectional area) will increase at that location. This means that there is a higher flow of carriers per unit cross sectional area, and this higher density of carrier flow will itself accelerate the electromigration phenomenon.
Additional background on electromigration and other thin film metallization issues can be found in G. Rao, Multilevel Interconnect Technology (1993); and in the three volume series by Wolf, Silicon Processing for the VLSI Era (1986).
Innovative Improved Interconnect Reliability Methods
This application discloses introducing a higher concentration of a hard-to-etch alloying agent (such as Cu in Al) in thin layers which are adjacent thicker layers having a lower concentration of the alloying agent. The use of layering improves interconnect reliability with minimal disruption of the metal etch process.
A thin, heavily alloyed layer combined with a thicker, less heavily alloyed layer presents less of a problem during etch than a single homogeneous layer, yet the advantages of the homogeneous layer can be achieved by subsequent thermal cycling and/or final sinter, if desired.
Higher doping concentrations can be achieved using the disclosed technique, and layers with higher doping concentrations can be strategically located. For example, they can be placed at both top and bottom interfaces, to greatly impede electromigration, or at critical interfaces where joule heating and current crowding are more severe and may lead to metal diffusion.
Advantages of the disclosed invention includes: